Imaging device and method for high-sensitivity optical scanning and integrated circuit therefor

ABSTRACT

An inspection system includes a CMOS integrated circuit having integrally formed thereon an at least two dimensional array of photosensors and providing an inspection output representing an object to be inspected. A defect analyzer is operative to receive the inspection output and to provide a defect report.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. patent applicationSer. 13/966,672 filed on Aug. 14, 2013, which is a continuation of U.S.patent application Ser. 13/351,542 filed on Jan. 17, 2012, issued asU.S. Pat. No. 8,536,506, which is a continuation of U.S. patentapplication Ser. No. 13/013,967 filed on Jan. 26, 2011, issued as U.S.Pat. No. 8,119,969, which is a continuation of U.S. patent applicationSer. No. 12/179,033, filed on Jul. 24, 2008, issued as U.S. Pat. No.7,897,902, which is a continuation of U.S. patent application Ser. No.11/532,549, filed on Sep. 18, 2006, issued as U.S. Pat. No. 7,417,243,on Aug. 26, 2008, which is a divisional of U.S. patent application Ser.No. 11/225,041, filed on Sep. 14, 2005, issued as U.S. Pat. No.7,129,509, on Oct. 31, 2006, which is a continuation of application Ser.No. 10/176,003, filed on Jun. 21, 2002, issued as U.S. Pat. No.7,009,163, on Mar. 7, 2006, which is a Non-Provisional of U.S.Provisional Patent Application No. 60/299,766, filed Jun. 22, 2001. Theentire disclosures of the prior applications are hereby incorporated byreference in their entirety.

FIELD OF THE INVENTION

The present invention relates generally to optical scanning systems andsensors, and specifically to scanning techniques employing twodimensional sensor arrays.

BACKGROUND OF THE INVENTION

Scanner systems for acquiring an image of an object, such as a printedcircuit board, are well known in the arts of imaging and automatedoptical inspection. Some conventional scanner systems include sensorscomprising a linear array of sensor elements. Other conventional scannersystems include sensors comprising a two dimensional array of sensorelements. Some systems employing two-dimensional array of sensorelements have been configured, for example, to operate in a time delayintegration (TDI) mode of operation to acquire and image of an object.Other system employing a two-dimensional array of sensor elements havebeen configured to acquire a sequence of non overlapping images of anobject.

TDI systems are well known in the art of optical scanning. In suchsystems, a sensor array is scanned over an object, such as a printedcircuit board, by moving either the array or the object in a directionperpendicular to the rows of the array. The scanning speed and an arrayclock are synchronized so that in each column of the array, multiplesensor elements in sequence capture light from the same point on theobject. Charge is accumulated between rows as the sensor array passesover the object so that sensor signals in each column are summed foreach point on the object, thereby providing an image of the object withenhanced signal/noise ratio.

Common TDI sensors are based on charge-coupled device (CCD) technology,which allows the sensor signals to be summed by transferring chargealong each column of the sensory array such that newly accumulatedcharge is added to charge having accumulated in previous rows of thecolumn. Other TDI systems based on photodiode arrays, such as CMOSsensor arrays, are also known in the art.

U.S. Pat. No. 5,750,985 and U.S. Pat. No. 5,909,026, the disclosures ofwhich are incorporated by reference, both describe sensors that can beemployed in a TDI type arrangement.

Applicants' copending U.S. patent application Ser. No. 10/141,988, filedon May 10, 2002 and entitled “Optical Inspection System Employing aStaring Array Scanner”, the disclosure of which is incorporated byreference, describes an inspection system employing a two dimensionalsensor array.

SUMMARY OF THE INVENTION

It is an object of some aspects of the present invention to provideimproved systems and methods for automated optical inspection (AOI).

It is a further object of some aspects of the present invention toprovide improved imaging techniques and devices employing twodimensional sensors.

In accordance with a broad aspect of the present invention, an at leasttwo dimensional array of photosensors formed on a CMOS integratedcircuit is employed to acquire images representing an object, such asimages of an electrical circuit. At least partially overlapping imagesare acquired, and pixels in the overlapping images, associated withcorresponding portions of the object, are added together to form acomposite image of the object. The composite image is particularlyuseful, for example, to inspect the object for defects. As used hereinthe term CMOS integrated circuit generally includes any suitableintegrated circuit comprising photosensors, such as photodiodes orphotogates, other than CCD type photosensors.

In preferred embodiments of the present invention, a two-dimensionalimaging device comprises a two-dimensional sensor array and a memory,having cells arranged in rows and columns that correspond to the rowsand columns of sensor elements in the array. The array and memory areconfigured to operate in a memory integration mode so as to provide acomposite image as the array scans over an object. In each cycle of thearray clock (i.e., each time the sensor array captures an image frame),the signal received by each of the sensor elements is digitized andadded to the value stored in one of the cells of the memory. A dynamicinput pointer indicates, for each row of the sensor array, the row inthe memory into which the signals from the sensor elements in that rowof the array should be added. The input pointer is advanced at eachcycle of the array clock in such a way that each memory cell receives asum of signals from multiple sensors in the same column of the array,captured as the sensors pass over the same point on the object. Adynamic output pointer is also updated at each cycle to indicate the rowof the memory in which integration has been completed, so that thememory integrated signal can be read out.

This use of dynamic input and output pointers enables the sensor array,memory and memory integration logic to be efficiently implementedtogether on a single chip, preferably using active pixel CMOS sensorelements. The dynamic pointer scheme also allows the direction ofscanning the array to be reversed simply by reversing the pointerdirection, so that the object can be scanned in a bidirectionalserpentine pattern, for example. This feature is particularly useful inAOI systems.

In some preferred embodiments of the present invention, the sensor arraycomprises color filters, so that the memory integrated image captured bythe array and memory comprises a color image. Preferably, the colorfilters are arranged so that successive rows of the array receive lightof different colors, typically in a repeating red-green-blue pattern.Alternatively, the color filters may be arranged so that successivegroups of rows receive light of different colors, for example, severalred rows, followed by several green rows, followed by several blue rows.Dynamic input and output pointers are provided for each color. Thedynamic pointer configuration and scan rate of the array over the objectmay be chosen to give either a color memory integrated image with fullresolution, equal to that of a comparable monochrome memory integratedimage, or a color memory integrated image that has reduced resolution,but whose throughput (i.e., speed of image capture) is equal to that ofa monochrome memory integrated image.

In some preferred embodiments of the present invention, the twodimensional imager is used in an AOI system, typically for evaluatingcharacteristics of objects such as printed circuit boards, flat paneldisplays, electronic assembly boards, and the like. The features of thememory integrated imager described above enable the system to operate athigh speed and with high sensitivity, in either a monochrome or colorimaging mode.

The present invention will be more fully understood from the followingdetailed description of the preferred embodiments thereof, takentogether with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic, pictorial illustration of a system for automatedoptical inspection (AOI), in accordance with a preferred embodiment ofthe present invention;

FIG. 1B is a simplified pictorial illustration that generally shows theoperation of the system of FIG. 1B in accordance with a preferredembodiment of the present invention;

FIG. 1C is more detailed illustration showing operation of a compositeimage generator shown in FIG. 1B;

FIG. 2 is a block diagram that schematically illustrates a twodimensional imaging device, in accordance with a preferred embodiment ofthe present invention;

FIG. 3 is a block diagram that schematically shows details of theimaging device of FIG. 2, in accordance with a preferred embodiment ofthe present invention;

FIG. 4 is a block diagram that schematically illustrates the use ofmemory pointers in the imaging device of FIG. 2, in accordance with apreferred embodiment of the present invention;

FIGS. 5-8 are timing diagrams that schematically illustrate theoperation of the device of FIG. 2, in accordance with preferredembodiments of the present invention;

FIG. 9A is a block diagram that schematically illustrates a sensor arrayused in a two-dimensional scanning color imaging device, in accordancewith a preferred embodiment of the present invention;

FIG. 9B is a block diagram that schematically shows details of detectorelements and memory cells in the device of FIG. 9A, in accordance with apreferred embodiment of the present invention;

FIG. 10 is a block diagram that schematically illustrates detectorelements and memory cells in a color two-dimensional imaging device, inaccordance with another preferred embodiment of the present invention;

FIG. 11A is a block diagram that schematically illustrates detectorelements and memory cells in a color two-dimensional imaging device, inaccordance with still another preferred embodiment of the presentinvention; and

FIGS. 11B and 11C are block diagrams that schematically illustratecontents of memory cells in the imaging device of FIG. 11A.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1A is a schematic, pictorial illustration of a system 20 forautomated optical inspection (AOI) of a printed circuit board 22, inaccordance with a preferred embodiment of the present invention. Board22 is shown here by way of example, and system 20 may similarly beadapted for inspection of other objects, such as flat panel displays,printed circuit boards loaded with electronic components, integratedcircuits, interconnect devices and moving webs. As used herein, the termelectrical circuit or board shall generally include any such suitablearticle to be inspected. The principles of the present invention, asdescribed in greater detail hereinbelow, may also be applied in otherareas of digital imaging, such as aerial surveillance.

System 20 captures images of board 22 using a camera 24, which is builtaround a CMOS integrated circuit imaging device 26 having an at leasttwo dimensional array of photosensors integrally formed thereon. Inaccordance with an embodiment of the invention, imaging device 26 isoperational in a memory integration mode of operation. An objective lens28 forms an image of board 22 on device 26 as camera 24 is scanned overthe surface of the board by a translation stage 32. Preferably, thecamera is scanned over the surface in a bidirectional serpentinepattern, so that the entire surface is imaged by the camera at a desiredlevel of resolution. Alternatively, board 22 may be translated whileholding camera 24 still, or both the board and camera may be translated,typically in mutually-perpendicular directions. A light source (notshown) illuminates board 22 as it is imaged by camera 24, preferably byproviding generally continuous illumination, or by providingnon-continuous illumination that is generally synchronized with a framerate of image frames acquired by imaging device 26.

A camera control unit 30 regulates the timing and operation of device26, and passes image data from device 26 to an image processor, oranalyzer, 34. The image processor analyzes the image data to locate andidentify faults, or defects, in board 22. In accordance with a preferredembodiment of the invention processor 34 comprises combinations of imageprocessing hardware and software such as are used in various AOI systemsavailable from Orbotech Ltd. of Yavne, Israel, including the Inspire9060™ and SK-75™ AOI systems. Alternatively or additionally, processor34 may comprise a general-purpose computer with suitable input circuitsand software for this purpose, hard-wired logic and/or a programmabledigital signal processor. For each board tested by system 20, processor34 outputs either a notification that the board is acceptable or anindication (such as a map) of a fault or faults found in the board, viaa display 36 or other output interface.

FIG. 1B is a simplified pictorial illustration that generally shows theoperation of system 20, in accordance with a preferred embodiment of thepresent invention, and to FIGS. 1C which is a more detailed illustrationshowing operation of a composite image generator seen in FIG. 1B.Imaging device 26 comprises an at least two dimensional array 40 ofphotosensors 42 integrally formed on a CMOS integrated circuit. Imagingdevice 26 generates an inspection output, typically in the form of imagedata 220, which corresponds to an object to be inspected such as board22. Defect analyzer 234 receives the image data from imaging device 26and provides a defect report 236 reporting defects on board 22, inresponse to analyzing the image data 220.

As seen in FIG. 1B, imaging device 26 is operative to acquire aplurality of images of board 22 during the scanning thereof. Fiverepresentative of sequentially acquired images, designated 240, 242,244, 246 and 248 respectively, are seen in FIG. 1B.

In accordance with an embodiment of the invention, images 240-248 aredigital pixel images that are sequentially acquired by imaging device 26during scanning a portion of board 22. Only five images are shown forthe sake of simplicity. Typically a much greater number of images isacquired. Each of the images 240-248 corresponds to a mutually offsetportion of board 22 such that each image of board 22 acquired by imagingdevice 26 at least partially overlaps another image. The mutual offsetbetween images may be as small as 1 pixel, although the mutual offsetbetween images may be greater.

Thus, as seen in FIG. 1B, image 240 is acquired by imaging device 26 ina first image frame. After board 22 advances relative to imaging device26 in the direction of arrow 250 by a distance of 1 pixel, image 242 isacquired in a second image frame. After board 22 further advancesrelative to imaging device 26 in the direction of arrow 250 by adistance of 1 pixel, image 244 is acquired in a third frame. After board22 further advances relative to imaging device 26 in the direction ofarrow 250 by a distance of 1 pixel, image 246 is acquired in a fourthframe. After board 22 further advances relative to imaging device 26 inthe direction of arrow 250 by a distance of 1 pixel, image 248 isacquired in a fifth frame. This sequence continues until at leastpartially overlapping images are acquired for an entire portion of board22.

A composite image generator 252 is operative to combine together each ofthe partially overlapping images generated by array 40, for exampleimages 240-248, and to supply composite image data 260 to analyzer 234.The composite image data 260 forms image 220 which has an improvedsignal/noise ratio compared to images 240-248. Image 220 is used bydefect analyzer 234 to detect defects in board 22.

In accordance with an embodiment of the invention, composite imagegenerator 252 is integrally formed on imaging device 26, although thisneed not be the case. As seen in FIG. 1B, each of images 240-248 is arelatively weak image of board 22, while image 220, which is the resultof combining images 240-248 comprises a significantly stronger image, asseen by the enhanced darkness of image portions corresponding toconductors 249.

The operation of composite image generator may be better understood fromFIG. 1C. Corresponding pixels 254 in each of images 240-248 are addedtogether to enhance pixel strength, that is to say improve signal tonoise. Pixels 254 in image 242 are added to corresponding pixels 254 inimage 240 to result in first composite image 274. It is seen that image242 is offset relative to image 240 and includes a sequentially addedrow of pixels 276. It is noted that for reasons of simplicity ofpresentation, due to orientation of images in FIG. 1C, the rows areactually seen as being columns. Pixels 254 to the left of row 276 infirst composite image 274 are darker than pixels in row 276.

Pixels in image 244 are added to corresponding pixels in first compositeimage 274 to result in second composite image 278. It is seen that image244 is offset relative to first composite image 274 and includes asequentially added row of pixels 280. Pixels to the left of row 276 insecond composite image 278 are darker than pixels in row 276, and pixelsin row 276 are darker than pixels in row 280.

Pixels in image 246 are added to corresponding pixels in secondcomposite image 278 to result in third composite image 282. It is seenthat image 246 is offset relative to second composite image 278 andincludes a sequentially added row of pixels 284. Pixels to the left ofrow 276 in third composite image 282 are darker than pixels in row 276,pixels in row 276 are darker than pixels in row 280, and pixels in row280 are darker than pixels in row 284.

Pixels in image 248 are added to corresponding pixels in third compositeimage 282 to result in fourth composite image 286. It is seen that image248 is offset relative to third composite image 282 and includes asequentially added row of pixels 288. Pixels to the left of row 276 infourth composite image 286 are darker than pixels in row 276, pixels inrow 276 are darker than pixels in row 280, pixels in row 280 are darkerthan pixels in row 284, and pixels in row 284 are darker than pixels inrow 288.

The above process is continued sequentially until a desired quantity ofcorresponding pixels are added together such that a gradient is formedin the composite image. At the end of each frame, line of pixelscomprising the result of adding together a plurality of pixels, isprovided as image data 260 (FIG. 1B).

It is a feature of some embodiments of the present invention that valuesadded together in the respective images 240-248 are digital values. Thedigital values are provided by at least one A/D converter associatedwith photosensors 42. An A/D converter may be associated with eachphotosensor 42. Optionally, each A/D converter is associated with aplurality of photosensors 42. For example, each A/D converter isassociated with a row of photosensors.

Preferred embodiments of the architecture, functionality and operationof imaging device 26 will be discussed hereinbelow in greater detail. Ingeneral, it is noted that imaging device 26 includes a plurality ofdigital registers which are operative to temporarily store the outputsof the A/D converters, digital memory, typically including an array ofmemory cells, storing image data provided by the array of photosensors,and a plurality of digital adders operative to add the outputs of thedigital registers to corresponding image data which is stored in thedigital memory.

Moreover, in accordance with embodiments of the invention the addingtogether of images, such as images 240-248, is performed on the fly on aline by line basis, and composite images are stored in a memory array ina wrap-around manner that dynamically changes as each new image 240-248is acquired and added to a previously stored composite image.

It is noted that images 240-248 seen in FIG. 1B generally correspond toimages formed on array 40. Typically these images are not stored betweenthe acquisition of successive image frames. As will be appreciated fromthe following detailed discussion of the operation of imaging device 26,each line in images 240-248 is retrieved and added to a correspondingline in a previously stored composite image, as described with referenceto FIG. 1C.

FIG. 2 is a block diagram that schematically shows the structure of amemory integration imaging device 26, in accordance with a preferredembodiment of the present invention. Device 26 is preferably fabricatedas a single integrated circuit (IC) chip, most preferably using a CMOSprocess. A sensor array 40 comprises a two-dimensional matrix of sensorelements 42, preferably active pixel sensors. In each frame (i.e., ateach cycle of the array clock), each element 42 generates a signalproportional to the light intensity incident thereon. The signals aretypically read out from the sensor array via column decoder 54 anddigitized by an array 44 of A/D (analog to digital) converters and arethen stored temporarily in an array 46 of registers, with one registerper column of sensor array 40. Alternatively, sensor elements 42 maycomprise digital pixel sensors, as described, for example, byKleinfelder et al., in “A 10,000 Frames/s 0.18 μm CMOS Digital PixelSensor with Pixel-level Memory,” presented at ISSCC 2001, which isincorporated herein by reference. In this case, the output of array 40is already digitized, and A/D converters 44 are unnecessary.

The digitized signal values held in register array 46 are summed by anarray 48 of adders with corresponding stored values in rows of a memory50, which typically comprises high-speed static or dynamic random accessmemory (SRAM or DRAM) or any other suitable type of memory. The resultsof the summation are stored back in the same row of the memory. Thisread/sum/store operation is typically performed for each cell in memory50 once per frame. It is repeated over a predetermined number of frames,each time adding in the signal from a different row in array 40, untilthe memory cell contains the sum of the signals taken from thepredetermined number of different elements 42 in the same column of thearray. The association of sensor elements with memory cells at eachcycle is controlled by a system of dynamic pointers, as described below.After the required number of summations of values from differentelements 42 have been performed for a given row of memory 50, theresults in that row are read out to an array 52 of output registers.These data are then clocked out of the registers to output ports 56 viaa column decoder 54, for readout to processor 34.

In a preferred embodiment of the invention, as seen in FIG. 2, a rowtiming block 58 is responsible for maintaining synchronization of theimage frame capture and readout by array 40, along with thecorresponding operations of A/D converter array 44, register array 46,adder array 48 and memory 50. The row timing is synchronized with thespeed of scanning camera 24 over board 22, as described below, such thatthose values from elements 42 that are added together at adders 48generally correspond to the same location on a board 22. Block 58controls the location of the dynamic pointers used in selecting the rowsof memory 50 for adding and readout and also includes the memory rowdecoder, in order to achieve a desired effect. Block 58 also controlsrow decoders and drivers 60, for reading out the signals from elements42 row by row in each frame, and for resetting array 40 at the end ofeach frame, via a reset control block 62.

FIG. 3 is a block diagram showing details of device 26, in accordancewith a preferred embodiment of the present invention. In this simplifiedembodiment, it is assumed that array 40 and memory 50 each comprise fourrows. For each column in array 40, there is a corresponding column ofcells in memory 50. For simplicity, only four of these columns areshown, as well.

Each sensor element 42 comprises a photodetector (or photosensor) 70,typically a photodiode or photogate, and an active amplifier 72 whichalso includes, for example, a select transistor (not shown). Theamplifiers are triggered by row select lines 74 to read out the chargestored by the corresponding photodetectors to column output lines 76.Photodetectors 70 are preferably designed for low capacitance, in orderto reduce the level of reset thermal (kTC) noise that they generate. Inaccordance with a preferred embodiment, each pixel also comprises areset circuitry (not shown), which is separately controlled by resetcontrol 62. Optionally, each sensor element may comprise a separatecharge storage element, such as a capacitor (not shown), to which chargeis transferred from the photodetector and held until it is read out ofthe array. As a further option, mentioned above, each sensor element maycomprise a built-in A/D converter (not shown). Other means known in theart may also be used to enhance the sensitivity and signal/noise ratioof array 40, such as the use of microlenses, integrated with the array,to focus light received by camera 24 onto photodetector 70 within eachsensor element 42.

Preferably, A/D converter array 44 comprises one A/D converter 78 percolumn of array 40. Optionally, an A/D converter may be associated witheach element 42. At each cycle of the row clock generated by row timingblock 58, converter 78 digitizes the signal from a successive element 42in its corresponding column of array 40. The digitized value is held ina register 80 in register array 46, until it is summed by an adder 82with the contents of a selected cell 84 in memory 50. The sums output byadders 82 are written back to the same cells in memory 50 from which theaddends were read out. Memory cells 84 are arranged in columnscorresponding to the columns of sensor elements 42 in array 40. In thepresent embodiment, cells 84 are arranged in four rows 86, correspondingto the four rows of elements 42 in array 40. The row whose cells 84 areto be read out for summing by adders 82 at each cycle of the row clockis determined by an input pointer 88. After a complete frame has beenread out of array 40, digitized and summed into the appropriate cells inmemory 50, pointer 88 is advanced to a new position for the next frame.As a result, each cell 84 in memory 50 receives the sum of the signalsgenerated by all four sensor elements 42 in the corresponding column ofarray 40.

An output pointer 92 is used to indicate the row 86 in memory 50 whosecells 84 contain the summed signals from all four of the sensor elements42 in the corresponding column of array 40. At each cycle of the rowclock, the contents of these cells are read out to registers 90 inoutput register array 52. After the contents of a row of cells have beenread out, the cells are reset to zero. Then, during the next frame,input pointer 88 is advanced so that the null contents of these memorycells are summed with the signals from the sensor elements in the firstrow of sensor array 40. In each subsequent frame, the pointers areadvanced, and summations are performed, until the cells again containthe sum of signals from all four rows of the sensor array and can againbe read out. Output pointer 92 is likewise advanced in each frame topoint to the next row of memory 50 that is to be read out.

FIG. 4 is a block diagram that schematically shows a single column 102of sensor array 40, and a single column 104 of memory 50, illustratingthe use of pointers 88 and 92, in accordance with a preferred embodimentof the present invention. In the embodiment shown in the precedingfigures, all columns are treated identically, so that the example shownhere in FIG. 4 is representative of the handling of the entire array. Anarbitrary object 100 is imaged onto column 102 of array 40 in foursuccessive frames, designated a-d respectively. For clarity ofillustration, the object is shown alongside column 102, rather thansuperimposed on it. The position of the object, which is in a translatedlocation respective of column 102 in each of four successive frames ofarray 40, is shown by successive bars 100 a, 100 b, 100 c and 100 d. Itwill thus be observed that the array clock of array 40 is synchronizedwith the speed of scanning the array over the object (or moving theobject under the array), so that the object advances by one pixel ineach successive frame. In other words, a point on object 100 that isimaged onto sensor element 42 a in the first frame is imaged onto thenext sensor element 42 b in the second frame, and so forth up to element42 d.

Input pointer 88 is set in each frame to point to the cell 84 in memory50 to which the signal from sensor element 42 a is to be added. Thelocation of the input pointer in each of the four successive frames(corresponding to bars 100 a-d) is shown in FIG. 4 by pointers 88 a, 88b, 88 c and 88 d, respectively. The signals from elements 42 b, 42 c and42 d are written to the succeeding cells in column 104, wrapping aroundback to the top of the column when the last cell (84 d) is reached. Thecorrespondence between sensor elements 42 and memory cells 84 in each ofthe four successive frames is indicated by solid arrows for the firstframe (bar 100 a), dashed arrows in the second frame (bar 100 b),dash-dot arrows in the third frame (bar 100 c), and dotted arrows in thefourth frame (bar 100 d).

Output pointer 92 is set in each frame to point to the cell 84 in memory50 to which the signal from sensor element 42 d is added. This cell willcontain, after the output of adder 82 is written back to the cell, thesum of the signals from all four of sensor elements 42 a-42 d in column102 for each successive pixel on object 100. The contents of this cellcan thus be read out to register 90 and then reset to zero. The positionof the output pointer in each of the four successive frames is shown inFIG. 4 by pointers 92 a, 92 b, 92 c and 92 d, respectively. When outputpointer 92 points to a given cell in one frame, input pointer 88 willpoint to that same cell in the next frame. Thus, on the next cycle ofthe array clock, the cell will begin to accumulate image data fromsensor element 42 a captured from a new pixel on the object.

FIG. 5 is a timing diagram that schematically shows timing signalsassociated with the operation of imaging device 26, in accordance with apreferred embodiment of the present invention. The figure illustratesthe operation of the device over two cycles of the array clock, i.e.,two frames. Each frame begins by resetting sensor array 40, to removeresidual charge from sensor elements 42, and then allowing the sensorelements to integrate charge over the remainder of the frame. Thesignals generated by the sensor elements are read out of array 40 row byrow, for rows 1 through N of the array. The signal values are digitizedand summed into memory 50, as described above.

After all the summations are complete, the summed data are read out ofcells 84 in the row 86 of memory 50 that is indicated by output pointer92. Pointers 88 and 92 are then advanced to their positions for the nextframe. As soon as all the rows of array 40 have been read out (evenbefore the pointers are advanced), the array can be reset, and theprocess begun over again.

Note that because of the order of reading out rows 1 through N of array40, the integration times of the rows are not uniform. Row 1 has theshortest integration time, while row N has the longest. (The timingpattern shown in FIG. 5 assumes that sensor elements 42 do not containany internal charge storage structure, such as an additional capacitor,or an internal A/D converter, which would allow the integration times ofall the rows to be equalized.) Since every cell 84 in memory 50 receivesand sums signals from all the sensor elements in the correspondingcolumn of array 40, however, the cumulative integration time is the samefor all pixels scanned by camera 24.

FIG. 6 is a timing diagram that schematically shows timing signalsassociated with the operation of imaging device 26, in accordance withanother preferred embodiment of the present invention. This embodimentis similar to that shown in FIG. 5, except that now the signal from eachsensor element 42 is read out of array 40 twice in each frame: once inforward sequential order from row 1 to N, and then again in reverseorder from row N to 1. This approach is useful in achieving betteruniformity of pixel response. The readout of sensor elements 42 ispreferably non-destructive, i.e., the signal is read out of each sensorelement without removing the charge from the element until the entirearray is reset.

FIG. 7 is a timing diagram that schematically shows timing signalsassociated with the operation of imaging device 26, in accordance withyet another preferred embodiment of the present invention. Reading outeach sensor element twice in each frame, as in the preceding embodiment,may reduce the speed of operation of device 26. Therefore, in thepresent embodiment, the direction of reading out the rows alternatesfrom frame to frame: once from row 1 to N, and the next time from row Nto 1. This approach provides improved pixel uniformity withoutcompromising readout speed. Preferably, array 40 comprises an evennumber of rows, in order to ensure uniformity of response over allpoints on object 22.

FIG. 8 is timing diagram that schematically shows timing signalsassociated with the operation of imaging device 26, in accordance withstill another preferred embodiment of the present invention. In thisembodiment, it is assumed that each sensor element in array 40 comprisesa capacitor, memory cell or other internal component capable of storingits charge or signal value after integration is terminated. Thus, auniform integration period can be set for all the elements of array 40.At the conclusion of the integration period, the charge accumulated bythe photodetectors in all the sensor elements is transferredsimultaneously to the respective internal storage components. Thesignals are then read out of the storage components, digitized (ifnecessary) and summed into memory 50 as described above. Meanwhile, thephotodetectors are reset and begin their next integration period, whilethe processing of the signals from the preceding integration period isgoing on.

Although the embodiments described up to now are directed to monochromeimaging, system 20 and imaging device 26 may also be adapted to capturecolor images of board 22. One approach for this purpose would be to usecolored strobe illumination (not shown), for example synchronized withthe array clock, in which a different color light (typically red, greenor blue) is used to illuminate the board in each successive frame, orfor several successive frames. In order to generate color images, memory50 must be divided into separate sections, for receiving and integratingthe signals corresponding to the different colors. Within each section,the data are summed and read out using input and output pointers insubstantially the same way as described above. As another alternative,described below with reference to the figures that follow, differentcolor filters are applied to separate rows of array 40. Of course,different color filters may also be applied to separate columns of thesensor array, but this option may be less desirable as it necessarilydetracts from the resolution of camera 24.

FIG. 9A is a block diagram that schematically illustrates a sensor array106 used in a two dimensional scanning color imaging device, inaccordance with a preferred embodiment of the present invention. Thisdevice is similar in most aspects to device 26, as shown and describedabove, and may be used in camera 24 in place of device 26. Therefore,only the salient differences, having to do specifically with capture ofcolor images, are described here.

In the preferred embodiment seen in FIG. 9A, the rows of array 106 aredivided into three groups: rows 110, which are configured to capture redlight; rows 112, which are configured to capture green light; and rows114, which are configured to capture blue light. Typically, each row orgroup of rows is overlaid by a suitable filter, which passes only therange of wavelengths that the particular row is supposed to detect, asis known in the art. Although each group of rows shown in FIG. 9A isshown as including three rows, each group may alternatively contain alarger or smaller number of rows. The number of rows need not be uniformamong the different color groups. For example, a greater number of rowsof one color (typically blue or green) can be used to compensate fornon-uniform sensitivity of the silicon sensor and to provide enhancedresolution of the overall image. Other color schemes, having differentnumber of groups or configured to capture different electromagneticradiation wavelength, may also be used.

FIG. 9B is a block diagram that schematically shows details of sensorelements in one column 102 of array 106 and memory cells 84 in acorresponding column 104 in memory 50, illustrating imaging based onarray 106, in accordance with a preferred embodiment of the presentinvention. At the top of the figure, object 100 is shown in each of ninesuccessive positions relative to column 102, labeled stage 1 throughstage 9, in a manner similar to that in which the successive objectpositions are shown above in FIG. 4. Each stage corresponds to asuccessive frame of array 106, i.e., to one cycle of the array clock.Object 100 is divided into pixels labeled I, II, III, . . . , XV, at aresolution corresponding to the resolution of array 106.

For each of stages I through IV, the figure shows the location of inputpointers 116 and output pointers 118, along with the summed signals heldin each memory cell 84. Three input pointers and three output pointersare provided, one for each color group. At each stage, the signals fromthe first red, green and blue pixels (R1, G1 and B1) are read into thememory cells indicated by the respective input pointers 116. The signalsfrom the remaining pixels in each color group are summed into the nextmemory cells in column 104, in a manner similar to that shown in FIG. 4.In this way, after three frames are collected in the memory (i.e., threestages have passed), a given memory cell contains the sum of the signalsfrom all three of the sensor elements in a given color group. This cellis indicated for readout by output pointer 118. Table I below lists thepixels whose color values are read out of column 104 at each stage:

TABLE I PIXEL OUTPUT FOR FIG. 9B Red Green Blue Stage output outputoutput 1 VII IV I 2 VIII V II 3 IX VI III 4 X VII IV 5 XI VIII V 6 XIIIX VI 7 XIII X VII 8 XIV XI VIII 9 XV XII IX

It will be observed that the red, green and blue outputs generated byarray 106 are out of registration by three rows (amounting to six rowsbetween the red and the blue outputs). The registration can be easilycorrected by adding a six-stage buffer for the red output and athree-stage buffer for the green output. These buffers can be providedin the two-dimensional color scanning imaging device itself or on aseparate chip in camera 24. Alternatively, the registration adjustmentmay be performed by processor 34 without prior buffering.

FIG. 10 is a block diagram showing detector elements and memory cells ina two-dimensional color scanning imaging device, in accordance withanother preferred embodiment of the present invention. In thisembodiment, red rows 110, green rows 112 and blue rows 114 areinterleaved in cyclic alternation, i.e., RGB/RGB/RGB/RGB. Alternatively,other interleaving patterns may be used, such as RGBG/RGBG, etc. At eachstage of operation of the device, each pixel on object 100 is imagedsimultaneously by a sensor element in each of rows 110, 112 and 114.Therefore, the red, green and blue color images are mutually-registeredwithout the need for buffering. The scanning of the imaging array overthe object and the array clock are timed so that from each frame to thenext, object 100 advances by a distance equivalent to one cyclic groupof rows, i.e., by three sensor elements. As a result, a scanning systembased on this embodiment will have high throughput but low resolutionwhen compared to the embodiment of FIGS. 9A and 9B.

As in the preceding embodiment, three input pointers 116 are provided,indicating cells 84 to which the first red, green and blue sensorsignals (R1, G1 and B1) are to be written at each stage. Three outputpointers 118 indicate the cells from which the summed pixel values areto be read out. For the present embodiment, in which the RGB cyclerepeats four times, the pointers return to their starting values afterfour frames, labeled stages 1, 2, 3 and 4, are completed.

FIGS. 11A, B and C are block diagrams that schematically illustratedetector elements and memory cells in a two-dimensional color scanningimaging device, in accordance with still another preferred embodiment ofthe present invention. Here, too, as in the preceding embodiment, red,green and blue rows of sensor elements are interleaved in the sensorarray, and the output pixel values in all three colors are in mutualregistration. In the present embodiment, however, full resolution ismaintained, at the expense of reduced speed and increased memory size.An imaging device that is configured to operate in the manner shown inFIGS. 11A-C can be reprogrammed in software (or firmware) to operate inthe mode of FIG. 10, as well, with higher throughput but reducedresolution.

The memory in the embodiment of FIGS. 11A-C comprises three columns 104for each column 102 of the sensor array. Preferably, columns 104 areorganized in three sections of memory cells 84: section 120 in FIG. 11A,section 122 in FIG. 11B and section 124 in FIG. 11C. The number ofmemory cells in each section is equal to the number of sensor elementsin the sensor array. The columns of memory cells in each section areconfigured to collect and buffer the sensor signals from all threecolors of every third pixel in object 100. Thus, in each successivestage, input pointers 116 for each color shift from one section to thenext so that, for example, the signal from the first blue sensor element(B1) is fed to section 124 in stage 1, section 122 in stage 2, andsection 120 in stage 3. These signal values belong respectively to pixelVII (stage 1), pixel VIII (stage 2) and pixel IX (stage 3). The signalvalues from the subsequent blue sensor elements (B2 and B3) are summedinto the memory cells that are displaced by three and six cells,respectively, from the blue input pointer. The green and red signals aretreated similarly. At each stage, one color is fed to each of thesections. According to this scheme, each of the memory cells is readfrom and written to only once in every three stages. During the othertwo stages, the cell simply holds its previous value. It is noted thatonly memory cells that are updated at the respective stage are shown,for clarity.

At each stage, output pointers 118 indicate three adjacent memory cells84 to be read out from one of the three sections. The output pointersalternate from section to section in each cycle. The three cells thatare read out in each stage contain the red, green and blue pixel values,respectively, of one of the pixels, which are read out of the memorysimultaneously. Since buffering is performed in the memory itself, noexternal buffering is required. Table II below lists the pixels whosevalues are read out at each stage:

TABLE II PIXEL OUTPUT FOR FIG. 9B Read from memory Stage Pixel outputsection: 1 I 124 2 II 122 3 III 120 4 IV 124 5 V 122 6 VI 120 7 VII 1248 VIII 122 9 IX 120

Although the preferred embodiments described above relate particularlyto detection of visible light, the principles of the present inventionmay similarly be adapted for detection of other types of radiation, andparticularly for infrared and ultraviolet light. Thus, the “colors”mentioned above should be interpreted more generally as referring todifferent wavelength bands.

It will thus be appreciated that the preferred embodiments describedabove are cited by way of example, and that the present invention is notlimited to what has been particularly shown and described hereinabove.Rather, the scope of the present invention includes both combinationsand subcombinations of the various features described hereinabove, aswell as variations and modifications thereof which would occur topersons skilled in the art upon reading the foregoing description andwhich are not disclosed in the prior art.

What is claimed is:
 1. An imaging device, comprising a two-dimensionalarray of sensor elements which generates image data corresponding to aplurality of successive overlapping image frames; an A/D converter whichconverts the image data into digital image data corresponding to theplurality of successive overlapping image frames; a plurality of digitalregisters which store the digital image data received from the A/Dconverter; a plurality of digital adders which add values of the digitalimage data stored in the digital registers to image data stored in adigital memory, thus forming a single composite image frame obtainedfrom the successive overlapping image frames, and storing the compositeimage frame in the digital memory, wherein each successive image frameis offset from an immediately preceding image frame by a spacing of notgreater than a single pixel, such that each addition, by the digitaladders of image data stored in the digital registers to image datastored in the digital memory adds a new single pixel line of data to thecomposite image frame.
 2. An imaging device, comprising atwo-dimensional array of sensor elements which generates image datacorresponding to a plurality of successive overlapping image frames; anA/D converter which converts the image data into digital image datacorresponding to the plurality of successive overlapping image frames; aplurality of digital registers which store the digital image datareceived from the A/D converter; a plurality of digital adders which addvalues of the digital image data stored in the digital registers toimage data stored in a digital memory, thus forming a single compositeimage frame obtained from the successive overlapping image frames, andstoring the composite image frame in the digital memory, wherein eachsuccessive image frame is offset from an immediately preceding imageframe by a spacing of greater than a single pixel, such that eachaddition, by the digital adders of image data stored in the digitalregisters to image data stored in the digital memory adds two or morenew pixel lines of data to the composite image frame.
 3. An imagingmethod comprising: generating image data corresponding to a plurality ofsuccessive overlapping image frames; converting the image data intodigital image data corresponding to the plurality of successiveoverlapping image frames; storing the digital image data in a pluralityof digital registers; adding values of the digital image data stored inthe digital registers to image data stored in a digital memory, thusforming a single, composite image frame obtained from the successiveoverlapping image frames; and storing the composite image frame in thedigital memory; wherein each successive image frame is offset from animmediately preceding image frame by a spacing of not greater than asingle pixel, such that the adding the values of the digital image datacomprises adding a new single pixel line to the composite image frame.4. The imaging method according to claim 3, wherein the generating theimage data comprises: repeatedly: acquiring an image frame of an objectusing an array of sensor elements and advancing one of the object andthe array of sensor elements with respect to the other of the object andthe array of sensor elements, by a distance corresponding to one pixel.